Cellular communications systems are well known for mobile subscribers, where fixed base stations serve handsets or vehicle-mounted units. Cellular systems are now being developed for fixed subscribers which will replace the conventional land line between a telephone subscriber and their local exchange by a wireless link. It is possible to provide fixed subscribers with a directional antenna which points towards the base station serving the area in which the subscriber is located (or some adjacent base station providing a better signal).
In cellular systems the coverage area is divided into cells, each cell served by a base station or base site which is allocated a channel or band of channels, i.e. a channel subset, selected from a limited number of available channels. In order to accommodate as many subscribers as possible it is important to make maximum use of the channels by frequently reusing them while maintaining an acceptable level of interference between neighboring cells operating on the same channels (known as co-channel interference). One of the known methods for increasing capacity is to divide each cell or base site into sectors, each sector being allocated a characteristic channel subset.
In some conventional sectored systems, channels are allocated such that the same channel is given to corresponding, similarly aligned, sectors in each base site or base site cluster. This causes a problem in that subscribers receive signals not only from the base site serving their particular sector but also from other base site sectors operating on the same channel. These systems still pose a problem when subscribers have narrow beamed directional antennas because the directional antenna is still aligned with both the wanted and unwanted (interfering) base site sectors.
There have been some attempts to reduce co-channel interference by varying the direction in which sectors or cells having the same channel face. European Patent Application EP 429 200 (GEC) illustrates some back-to-back and partially rotated sectored base site arrangements for microwave broadcast systems. U.S. patent application U.S. Pat. No. 4,352,110 provides a cluster of outward-facing unsectored directional base sites arranged in concentric rings, which requires a prohibitively large number of base sites to serve an area. Another patent application, WO 92/22148, illustrates a five base site, sixteen sectored arrangement. This specification acknowledges that subscribers could be directional, but does not fully exploit the subscribers' directional properties in allocating channels. Furthermore, patent application EP No. 435,283 (NEC) shows several arrangements of sectored sites, one of which rotates an entire cluster of base sites by 120.degree. clockwise or anti-clockwise.
Most of the systems shown in the prior art are concerned with serving subscribers who are equipped with omnidirectional antennas, which receive signals equally from all directions. The allocation of channels to base sites in these systems is therefore restricted to prevent strong unwanted interfering signals. These allocation patterns may be applied to systems which serve directional subscribers, but they fail to exploit the improvements in channel re-use which directional subscribers allow.
It is an object of the present invention to minimise both co-channel interference and the number of base sites required in a cellular communications system which serves directional subscribers.